Light wave is an electromagnetic wave consisting of the electric and magnetic fields. As it propagates, the electric and magnetic fields oscillate on a transverse plane normal to the propagation direction with a specific oscillation pattern. In general, the polarization is a field which is parallel to the electric field vector E. Therefore, the state of polarization is referred to the oscillation pattern of the electric field on the transverse plane. The state of polarization falls into three categories in terms of the direction and phase of two mutually-orthogonal components of the electric field vectors E. The categories are linear, circular, and elliptical polarization, respectively.
The state of polarization in changed when light propagates through a birefringence medium which has different refractive-indices between two orthogonal eigen axes. The amount of birefringence, or the index difference, is the characteristics of the material itself, however it is affected by external perturbations such as stress, strain and temperature. Silica material is inherently birefringence-free because of its amorphous nature. However, optical fibers that are made of silica tend to exhibit non-negligible birefringence owing to the internal stress as well as non circular-symmetric geometry. Additional birefringence can be also induced by external lateral stress or bending. The change of such birefringence by external perturbations which is often non-deterministic can cause severe problems in fiber-optic applications such as optical communications and sensors. The effects include polarization-induced signal fading or degradation due to the change of polarization state. It is therefore very important in fiber-optic applications to maintain or control the polarization state of light. A polarization controller is an apparatus used to convert an input polarization state to an arbitrary output polarization state and, therefore, a key element in lab experiments, fiber-optic sensors, optical communications, and especially for a system which uses highly polarization dependent optical devices. For example, high-speed optical system employs lithium niobate(LiNbO.sub.3) as an external modulator to reduce wavelength chirping that comes in at directly modulated light source. In this case, due to the high-polarization dependence of the modulator, matching the polarization state to the birefringence axis of the modulator is essential to get the best performance. In matching the polarization state between a laser diode("LD") and the external modulator, polarization maintaining("PM") fiber is generally used to connect the LD to the external modulator. However, this requires complex process of aligning polarization axis of fiber to those of the LD and the external modulator.
The principle of the polarization controller is that desired polarization state is obtained by using appropriate phase retarders which can transform a state of polarization("SOP") to another SOP. Two quarter-wave plates can be used for the phase retarders.
FIG. 1 illustrates the change of SOP on Poincare Sphere which is generally accepted way to describe the SOP. The convention is that a linear polarization state can be represented by a point "c" located on the equator, and circular polarization by a point "d" located on the pole of the Poincare sphere. A point on the sphere corresponds to a SOP. Since two orthogonal axes of a coordinates can describe any point on the sphere, a point on the sphere can be moved to another point by rotating the two orthogonal axes of the coordinate. This means that any SOP of light can be transformed to any other SOP by means of rotating the axes. It is well-known that two quarter-wave plates that are used in a polarization controller, for example bent fiber loops for making phase retardation by inducing birefringence therein, can perform the above described two rotating axes.
When two quarter-wave plates are used for a polarization controller, the respective azimuthal and polar angles of a point on the sphere can be rotated by rotating two axes of the sphere, namely, the optical axes of two quarter-wave plates. Therefore, input SOP "a" can be transformed to a desired output SOP "b" as shown in FIG. 1.
FIG. 2 schematically shows a conventional optical fiber polarization controller according to a prior art.
The polarization controller shown in FIG. 2 is disclosed by H. C. LeFevre, in Electronics Letters, Vol. 16, No. 20, September 1980. Referring to FIG. 2, a length of single mode optical fiber 1 is wound on a bobbin 10 with a predetermined diameter. The bending gives birefringence to the optical fiber by bending-induced stress, which makes two birefringence axes, parallel and perpendicular to bobbin 10. At an appropriate diameter, the induced birefringence makes a quarter-wave plate for a given optical wavelength. Since the birefringence principal axis rotates as the rotation of bobbin 10 to R-direction, the SOP of input light P.sub.in can be controlled to a desired SOP polarization state in output light P.sub.out. With this bobbin, the polarization controller can not avoid comparably large volume, making it hard to be mounted on a common circuit board.
FIG. 3 is a cross sectional view showing the application of other polarization controller of prior arts. This kind of polarization controller can give small size compared with the above described prior art. Referring to FIG. 3, the polarization controller has a screw 32 which can contact the outer surface of optical fiber 31. In the polarization controller of FIG. 3, the polarization state is controlled by the birefringence, induced from the mechanical stress which is applied to the optical fiber by screw 32. In principle, there should be means for acting as two orthogonal rotating axes to control the polarization state as described in FIG. 1. The means to achieve this action is the screw that presses the optical fiber from different directions with different force, which corresponds to the two orthogonal axes of the above polarization controller. For example, as shown in FIG. 3, after the stress applied in X-Y direction is released, other stress is applied in X'-Y' direction. This single controlling means may cause difficulty in controlling the polarization. The problem with such polarization controller is that the reliability of the polarization controller is significantly affected by the squeezing of the fiber, because the squeezing can damage the jacket of the fiber and fiber itself. Moreover, the mechanically squeezed jacket may not recover its original form so that the stress still remains in the fiber, which results in uncontrollable situation.